1. Field of the Invention
The present invention relates to a radiation imaging apparatus and a control method for the radiation imaging apparatus.
2. Description of the Related Art
Recently, in the field of digital radiation imaging apparatuses, instead of an image intensifier, a large-area flat panel sensor based on a 1× optical system using photoelectric conversion elements has been widely used for the purpose of increasing resolution, decreasing volume, and suppressing image distortion. Imaging apparatuses using photoelectric conversion elements include amorphous-silicon-type apparatuses, CCD type apparatuses, and CMOS-type apparatuses. Imaging elements using amorphous silicon semiconductors on a glass substrate allow the apparatus to easily form a large-area imaging apparatus. On the other hand, as compared with a single-crystal silicon semiconductor substrate, amorphous silicon makes it difficult to microfabricate a semiconductor substrate on a glass substrate. As a consequence, the larger the capacitance of an output signal line, the more insufficient the semiconductor characteristics. Although a CCD-type imaging apparatus is of a completely-depleted type and has high sensitivity, the use of this apparatus as a large-area imaging apparatus will lead to an increase in the number of transfer stages for electric charge transfer, resulting in power consumption as much as 10 times that of an apparatus using CMOS-type imaging elements. That is, this technique is not suitable for a large-area imaging apparatus.
Japanese Patent Laid-Open No. 2002-344809 discloses a large-area flat panel type sensor, which implements large-area imaging by using CMOS-type imaging elements as photoelectric conversion elements, and more specifically by tiling rectangular imaging elements which are rectangular CMOS-type photoelectric conversion elements cut out from a silicon semiconductor wafer. A CMOS-type imaging element is capable of fast readout owing to microfabrication as compared with amorphous silicon, and hence allows the apparatus to obtain higher sensitivity. In addition, a CMOS-type imaging element is known as being highly advantageous when implementing a large-area, flat-panel-type sensor. This is because this element is free from the problems of using a number of transfer stages for electric charge transfer and power consumption, unlike a CCD imaging element, and hence facilitates the implementation of large-area imaging.
In addition, Japanese Patent Laid-Open No. 2006-319529 discloses an arrangement using a pixel addition circuit in a CMOS-type imaging element and a sensitivity switch.
Consider a CMOS-type imaging element which implements both pixel addition and sensitivity switching. In this case, when, for example, this element is driven in a high sensitivity mode, the sensitivity switch is turned off, and one end of a dynamic-range expansion capacitor is opened to become a floating capacitor. The electric charge accumulated in this floating capacitor is unstable, and hence unstable voltages are generated in the circuit of the CMOS-type imaging element. When unstable voltages are generated in the circuit of a CMOS-type imaging element in the moving-image capturing operation, a small amount of leakage between the gate and source of each MOS transistor in the circuit of the CMOS-type imaging element becomes unstable, resulting in random noise affecting each frame.
In addition, a CMOS-type imaging element has an offset value, and each pixel outputs a non-zero value as an optical signal even without the application of light. There is available a method of defining optical-signal data acquired without the application of light as a fixed-pattern noise (FPN) pattern of a CMOS-type imaging element and subtracting the FPN pattern from optical-signal data obtained when acquiring a moving image. However, since the electric potential of each floating portion for each moving-image capturing operation changes with time, the electric potential of the floating portion in the CMOS-type imaging element which has acquired an FPN pattern before the imaging operation differs from the electric potential of the floating portion which is set when a moving image is actually acquired. This causes a difference between the FPN pattern and a noise component derived from the unstable voltage of the floating portion in moving image data subjected to FPN correction, resulting in a failure to perform a proper FPN correction.